Along with the development of electronic devices, lighting emitting diode (LED) is widely used for its high light intensity, direct current (DC) driving ability, long life and environmental-friendness.
At present, LED power supply mainly has two dimming methods. One is silicon controlled rectifier (or called TRIAC) dimming and the other is pulse width modulation (PWM) dimming. Compared to TRIAC dimming, PWM dimming has the advantage of no LED shimming and thus is preferred.
In order to decrease power dissipation of the LED power supply with PWM dimming, PWM dimming signal is used to control both the LED and the power supply. When the LED is turned off, the power supply is also shut off by the PWM dimming signal. FIG. 1 illustrates a prior art LED power supply 10 with PWM dimming. As shown in FIG. 1, LED power supply 10 comprises a boost converter 101, a LED load 102, a feedback circuit 103, a feedback loop control circuit 104 and dimming switches S1, S2 and S3. Feedback loop control circuit 104 comprises a double loop control circuit, which receives a feedback signal VFB from the feedback circuit 103, a reference signal VREF and an inductor current sensing signal ISENSE from the boost converter 101, and a driving signal generator 105 of the feedback loop control circuit 104 generates a control signal CTRL which is used to control a power switch of the boost converter 101. When the PWM dimming signal transits from logic HIGH to logic LOW, dimming switches S1, S2 and S3 together with switch S4 in control circuit 103 are turned OFF, and driving signal generator 105 is disabled, in order to decrease the power dissipation and improve efficiency.
However, in applications with wide range of dimming ratio, the dimming ratio or called the duty cycle of the PWM dimming signal may be very small, and in one dimming cycle, the inductor current even is unable to increase from zero to the desired level, and thus the power supplied to the LED load is not high enough. If this situation maintains in a period, the output voltage of the converter will decrease and makes the LED load functions abnormally.